A power supply unit on board an aircraft as a substitute for a main engine, an auxiliary power unit, a ram air turbine or an NiCd battery is known from EP 957 026 A2. A fuel cell serves here to generate direct current, whereby used air from the aircraft air-conditioning unit or aircraft external air is used for the air supply to the fuel cells. Water for the water supply on the aircraft is obtained from the fuel cell exit air, whereby the fuel cell exit air is then carried away to the aircraft surroundings, which also applies to the hydrogen emerging from the fuel cell. Generation of water by means of a water condenser arranged in the exit air flow can take place as an advantageous secondary effect. The arrangement of the fuel cell module is provided in the aircraft tail.